As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semi-conductor wafers. The basis for these techniques is the notion that a subject may be examined by analyzing the reflected energy that results when a probe beam is directed at the subject. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in magnitude are analyzed. Scatterometry is a related technique that measures the diffraction (optical scattering) that the subject imparts to the probe beam.
Techniques of this type may be used to analyze a wide range of attributes. This includes film properties such as thickness, crystallinity, composition and refractive index. Typically, measurements of this type are made using reflectometry or ellipsometry as described more fully in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference. Critical dimensions (CD) including line spacing, line width, wall depth, and wall profiles are another type of attributes that may be analyzed. Measurements of this type may be obtained using monochromatic scatterometry as described in U.S. Pat. Nos. 4,710,642 and 5,164,790 (McNeil). Another approach is to use broadband light to perform multiple wavelength spectroscopic reflectometry measurements. Examples of this approach are found in U.S. Pat. No. 5,607,800 (Ziger); U.S. Pat. No. 5,867,276 (McNeil); and U.S. Pat. No. 5,963,329 (Conrad). Still other tools utilize spectroscopic ellipsometric measurement. Examples of such tools can be found in U.S. Pat. No. 5,739,909 (Blayo) and U.S. Pat. No. 6,483,580 (Xu). Each of these patents and publications are incorporated herein by reference.
As shown in FIG. 1, a typical optical metrology tool includes an illumination source that creates a mono or polychromatic probe beam. The probe beam is focused by one or more lenses to create an illumination spot on the surface of the subject under test. A second lens (or lenses) and an aperture image the illumination spot (or a portion of the illumination spot) to a detector. The detector captures (or otherwise processes) the received image. A processor analyzes the data collected by the detector. For operation as an ellipsometer, the optical metrology tool includes a polarizer that imparts a known polarization state to the probe beam. A second polarizer, known as an analyzer is used to determine the polarization state of the probe beam after reflection by the subject.
Over time, as the sizes of the features on semiconductor wafers decreases, there is an increasing need to use smaller and smaller illumination spots. For the reflectometry case, measurement can be effectively recorded when the probe beam is directed normally to the subject (normal incidence). This mitigates the difficulty of producing small spot sizes, since normal incidence inherently minimizes the size of the illumination spot. The ellipsometry case is more difficult because sensitivity to film attributes improves as angle of incidence increases. As a result, measurements of this type are typically made using a relatively high angle of incidence, usually around seventy degrees. This spreads the illumination spot into an ellipse whose major radius is equal to its minor radius multiplied by 1/cos(θ) (where θ is the angle of incidence). At seventy degrees, the resulting illumination spot is almost three times as long as it would be at normal incidence.
One approach for performing ellipsometric measurements with small spot sizes was developed by the assignee herein. In these systems, a high numerical aperture lens was used to create a spread of angles of incidence with a generally normal incidence beam. Such a system using broadband light is disclosed in U.S. Pat. No. 5,596,411 (Fanton).
More recently, it has been proposed to operate a spectroscopic ellipsometer in a normal incidence mode to measure critical dimensions. More specifically, while normal incidence ellipsometry is insensitive to general thin film parameters, it had been known for some time that such a configuration could be used measure surface anisotropy. (See, “Reflectance-difference Spectroscopy System for Real-time Measurements of Crystal Growth,” Aspnes, et. al., Applied Physics Letters, 52 (12) Mar. 21, 1988, page 957.) By extension, the use of such systems for monitoring critical dimensions has been discussed. (See, “Normal Incidence Spectroscopic Ellipsometry for Critical Dimension Monitoring,” Huang, et. al, Applied Physics Letters, 78 (25) Jun. 18, 2001, page 3983.) In the latter article, it was shown that changes in polarization state for a near normal incidence beam can be attributed virtually entirely to the surface structure rather than the underlying thin film layers.
Operation at normal incidence produces the smallest possible spot size and is an effective method for measuring critical dimensions. Unfortunately, in cases where thin film measurements are also required, normal incidence measurement has been less effective. For these reasons, there is a need for metrology systems that can accurately measure both surface structure and the parameters of the thin films underlying the structure. Further, it is important that these measurements be made over a relatively small spot size.